1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image to a recording medium with heat and pressure and an image forming apparatus including the fixing device.
2. Description of the Related Art
As a fixing device for use in an electrophotographic image forming apparatus such as a copying machine and a printer, a fixing device of heat-roller fixing type has been in wide use. The heat-roller fixing-type fixing device includes a pair of rollers (a fixing roller and a pressure roller) that is brought into contact with each other under pressure. By means of a heating section composed for example of a halogen heater, which is placed in each of or one of the pair of rollers interiorly thereof, the pair of rollers are heated to a predetermined temperature (fixing temperature). With the pair of rollers kept in a heated state, a recording medium such as a recording sheet, having formed thereon an unfixed toner image, is fed to a pressure-contact region of the pair of rollers (i.e., a fixing nip region). Upon the recording sheet passing through the pressure-contact region, the toner image is fixed into place under application of heat and pressure.
Incidentally, a fixing device for use in a color image forming apparatus generally employs an elastic roller constructed by forming an elastic layer made for example of silicone rubber on a surface layer of the fixing roller. By designing the fixing roller as an elastic roller, it is possible for the surface of the fixing roller to become elastically deformed so as to conform to irregularities of the unfixed toner image, wherefore the fixing roller makes contact with the toner image so as to cover the surface of the toner image. This makes it possible to perform satisfactory thermal fixation on the unfixed color toner image that is larger in toner adherent amount than a monochromatic toner image. Moreover, by virtue of a deflection-releasing effect exerted by the elastic layer in the fixing nip region, it is possible to provide enhanced releasability for a color toner that is more susceptible to occurrence of offset than a monochromatic toner. Further, since the fixing nip region is convexly curved in an upper direction (i.e., on a fixing roller side) so as to define a so-called reverse nip configuration, it is possible to attain higher paper-stripping capability of the recording sheet. That is, a paper stripping action of the recording sheet can be produced without using a stripping portion such as a stripping pawl (self-stripping action), wherefore image imperfection caused by the provision of the stripping portion can be eliminated.
Incidentally, in such a fixing device provided in a color image forming apparatus, it is necessary to make a nip width of a fixing nip region wide in order to correspond to increase in speed. One available method of increasing the fixing nip width is to increase the thickness of the elastic layer of the fixing roller or the diameter of the fixing roller. However, in a fixing roller having an elastic layer, the elastic layer can not sufficiently conduct heat, thus, in a case where a heating section is provided inside the fixing roller, there is a problem that a temperature of the fixing roller is not followed when a process speed is increased. On the other hand, when a diameter of the fixing roller is increased, there is a problem that it takes longer time to warm up or power consumption is increased.
As a fixing device provided in a color image forming apparatus to solve such problems, Japanese Unexamined Patent Publication JP-A 10-307496 (1998) discloses a fixing device in a belt fixing system that is configured so that a fixing belt is supported around a fixing roller and a heating roller and the fixing roller and a pressure roller are brought into pressure-contact with each other with the fixing belt interposed therebetween. In the fixing device in a belt fixing system, since the fixing belt with small heat capacity is heated, it takes short time to warm up and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus making it possible to provide a thick elastic layer with low hardness made of sponge rubber and the like and to secure a wide nip width.
Furthermore, Japanese Unexamined Patent Publication JP-A 2002-333788 discloses a fixing device in a planar heat generating belt fixing system with a heating section as a planar heat generating element. In the fixing device in a planar heat generating belt fixing system, when heat capacity of the heating section is reduced, the planar heat generating element as the heating section directly generates heat at the same time, thus a thermal response speed is also enhanced compared to a system in which a heating roller is heated indirectly using a halogen lamp or the like and it is possible to attain further shortening of a time for warm up and more energy saving.
However, in a fixing system using a resistance heat generating element as the planar heat generating element, a member with small heat capacity is used so that a surface temperature is determined by a balance between transmitted heat and radiated heat, thus heat radiation volume from both ends of the roller is increased when heat is generated. Accordingly, the temperature of the both ends of the planar heat generating element is lower than that of a center part and it is difficult to obtain uniform temperature distribution over the all areas in a longitudinal direction. As a result, when such a fixing device is applied to an image forming apparatus such as a copier and a printer, variance is generated in a toner fixing temperature and the printing quality is deteriorated.
As a fixing device to solve such problems, Japanese Unexamined Patent Publication JP-A 2003-57984 discloses a fixing device in a DH fixing system, in which, in a fixing system using a fixing roller and a pressure/fixing roller, in order to shorten a time for warm up and uniform distribution of a temperature in an axial direction of the fixing roller surface, a resistance heat generating layer is provided in a lower part of a surface layer of the fixing roller, and a fixing roller is further provided, that transmits, directly to the surface layer, heat generated by making thickness of the resistance heat generating layer have distribution in an axial direction and electrifying the resistance heat generating layer. In the fixing device in a DH fixing system, heat is transmitted to the surface without interposing a core metal with large heat capacity, thus making it possible to shorten a time for warm up, in addition, to reduce unevenness of the surface temperature by generating a large amount of heat matching to heat radiation volume from both ends of the roller.
The fixing device in a planar heat generating belt fixing system described above has the following problems. That is, when the planar heat generating element does not have a self-temperature-controlling function (positive resistance temperature characteristic), during continuous printing of recording sheets having different sizes, a temperature of a non-sheet passing part is excessively increased in the heating roller, thus a step of reducing the temperature of the heating roller to an appropriate level is required, and it takes longer time to perform printing, which reduces productivity significantly. In addition, since a temperature of the both ends in a longitudinal direction of the planar heat generating element is excessively increased, a life of a fixing member is shortened. In order to prevent this, there is considered a method for divisionally controlling the resistance heat generating element, but a detecting member and a control member corresponding to individual heat generating elements are required to divisionally control the resistance heat generating element, which are expensive and complicated.
FIGS. 10A and 10B is a view showing distribution of a temperature, distribution of heat radiation volume, distribution of electric resistance, and distribution of thickness with respect to a length from an end in a longitudinal direction in a heat generating layer provided in a planar heat generating element in a DH fixing system. FIG. 10A shows behavior in a case where a planar heat generating element having a positive resistance temperature characteristic is used and FIG. 10B shows behavior in a case where a planar heat generating element having no positive resistance temperature characteristic is used. In a fixing device in a DH fixing system, in a case where a planar heat generating element has a positive resistance temperature characteristic, when the planar heat generating element is electrified in a longitudinal direction, electric resistance of both ends in the longitudinal direction thereof is increased and heat generation at an end in the electrifying upstream side is accelerated. Then, due to the positive resistance temperature characteristic, the electric resistance of an area of the end in the electrifying upstream side is increased suddenly and electrifying to the planar heat generating element is stopped. Accordingly, the distribution of a temperature in a surface of the planar heat generating element has a state where a temperature in the area of the end in the electrifying upstream side is high.
Furthermore, in the fixing device in a DH fixing system, when the planar heat generating element does not have a positive resistance temperature characteristic, as shown in FIG. 10B, the temperature distribution in a surface of the planar heat generating element is uniform, but voltage of the planar heat generating element is divided in the electrifying direction, thus a temperature rising rate becomes slow even when heat radiation volume is capable of being controlled. Even when electric resistance is adjusted (low resistance) in view of the divided voltage, it is very dangerous to directly supply large current to a fixing roller that is rotating.